The present invention relates to a method to permit the selective plating of a metal substrate, such as an electrical contact. The selective plating thereof, such as by plating with a precious metal, is achieved herein by selectively removing a plating resist covering said substrate. Such removal is accomplished by matching or coupling the laser wavelength, preferably one operating in the U.V. range, such as an excimer laser, with the plating resist and metal substrate.
A preferred embodiment of this invention is the selective plating of electrical terminals. Typically, such terminals are stamped and formed from metal strip and are attached to a carrier strip which is useful for strip feeding the terminals through successive manufacturing operations. One necessary manufacturing operation involves plating; i.e., electroplating, the electrical contact surfaces of the strip fed terminals with precious metal or semi-precious metal, such as gold or alloys thereof. Such metals are characterized by good electrical conductivity and little or no formation of oxides that reduce said conductivity. Therefore these metals, when applied as plating, will improve conductivity of the terminals. However, the high cost of these metals has necessitated precision deposition on the contact surfaces of the terminals, and not on surfaces of the terminals on which plating is not necessary. The present invention achieves this result by a method not found nor reported on in the prior art.
Examples of conventional selective plating practices are found in U.S. Pat. Nos. 4,555,321 and 4,473,445 where such patents disclose apparatus for the continuous plating of surfaces of electrical terminals. Said apparatus includes a rotating mandrel for guiding said terminals through a plating solution.
Technology has advanced to the stage that laser beams have been employed to improve metal substrate surfaces for subsequent plating. In co-pending application Ser. No. 133,779, now U.S. Pat. No. 4,832,798 and owned by the assignee herein, a technique is taught whereby the porosity of a nickel plated substrate is significantly reduced by a laser beam to permit a reduction in the level of precious metal plating needed on such nickel to produce a good contact surface.
U.S. Pat. No. 4,348,263 to Draper et al and directed to a process for surface melting of a substrate prior to plating, teaches a method of making an electrical contact by the steps of applying a first protective layer to a substrate, subjecting said protective layer and a portion of said substrate to melting by means of an electron beam or laser prior to the deposition. A related work by Draper, published in the Gold Bulletin, 1986, 19, entitled "Laser Surface Alloying of Gold," contains an illustrated showing on the mechanism of laser surface alloying by the use of focused laser pulsing.
Reports have appeared in the literature regarding attempts at laser ablation of polymer coatings on metals, and regarding methods of multi-shot removal of polymer coatings on non-metals. R. Srinivasan et al, in the JAP 59, 3861 (1986) and JVST, Bl, 923 (1983) describe, for example, the use of excimer laser wavelengths which are strongly absorbed directly in the polymer itself to achieve removal of polymer by chemical bond-breaking or heating to vaporization, or a combination of both. However, the authors found that polymer ablation occurs when the laser light is absorbed within about the first 0.2 micron or less of the polymer surface. Then only that polymer material within the characteristic absorption depth was removed. In order to remove a thicker polymer film, such as is necessary for most electroplating requirements, multiple laser shots would be required. The use of multiple shots is much less desirable than single shot removal. One problem associated with the method of Srinivasan et al, wherein the laser light is directly absorbed in the polymer, is that choosing a laser wavelength too strongly absorbed in the polymer necessarily implies a small absorption depth and small thickness removed. On the other hand, choosing a wavelength too weakly absorbed in the polymer precludes depositing sufficient energy per unit volume of polymer to achieve ablation. The compromise value between these extremes dictates that no more than about 0.3 micron per pulse can be removed in the best case. Cole et al, in Mat. Res. Soc. Symp. Proc. 72, 241 (1986), concur with Srinivasan et al in this finding. The above process represents the current state of the art on excimer laser ablation of polymers.
However, the above description ignores another serious factor which applies only to removal of films deposited on metal substrates. Such problem has not shown up in some of the work, such as by Srinivasan et al, because they were concerned with polymer films on semiconductors or insulators (or free-standing), or in other cases because the workers were not sensitive to a very thin residual polymer layer remaining after the attempted laser ablation. However, such residual layer is severely detrimental to electroplating.
In U.S. Pat. No. 4,671,848 to Miller et al, a method for the removal of a dielectric coating from a conductor, by means of a focused, high energy radiation source, is taught. More particularly, in said method a laser source is focused to a point above the coating which results in a plasma or ionized region being formed. As a consequence, the coating is removed in a preselected region on the underlying conductor. In other words, the laser ablation depends on absorption of laser light by ionized air or other plasma and transmission to the dielectric. A difficulty of this method is the ability to control and adjust the air breakdown so as to ensure there is no damage to the conductor, i.e. underlying substrate, and to achieve removal of the residual layer. Another difficulty is that only a small area corresponding to the tight focus region can be removed on each shot. Miller et al state that extended areas are to be abated by multiple shots while moving the workpiece, or the laser focus.
In the Miller investigation cited above, the reason for the residual layer is believed to be a fundamental boundary condition on electromagnetic waves outside a metal surface. It is known that as a result of such condition, there is a decrease of the transverse electric field to small values as the metal surface is approached, thus ensuring insufficient direct heating of the polymer immediately adjoining the metal. This effect is further aggravated by the heat-sinking effect of the cold metal substrate surface underlying the polymer. To compensate, if a polymer is selected to have a very high absorption coefficient at the high wavelength of the laser to combat this effect, only a thin layer comparable to the optical absorption depth is removed with a single laser shot. As a consequence, multiple shots are required.
The present invention avoids these and other difficulties by the unique selection or matching of parameters, namely an excimer laser, a compatible polymer and substrate adapted to the wavelength thereof. The manner such selection brings these features together will become apparent from a reading of the specification which follows.